To decrease PM contained in exhaust gas, a diesel engine (hereinafter, engine) is provided with a DPF in an exhaust pipe line. The DPF collects PM such as soot contained in exhaust gas, and discharges exhaust gas in which PM is decreased to air. When PM collected in the DPF increases, a filter performance is degraded, and thus a regeneration that combusts the collected PM is performed in the DPF. The regeneration includes a natural regeneration in which PM accumulated when a temperature of exhaust gas is high naturally combusts, and a forced regeneration performed when an amount of PM accumulation exceeds a predetermined reference value. In the forced regeneration, an operating condition of an engine is adjusted to raise a temperature of exhaust gas, and PM is forcibly combusted by performing an external dozing that injects fuel at a previous stage of the DPF, or an internal dozing that injects fuel to the inside of a cylinder of the engine. Further, the forced regeneration includes an automatic forced regeneration and a manual forced regeneration. When the amount of PM accumulation further increases to exceed a reference value used when the automatic forced regeneration is performed, and the DPF is likely to be blocked, a warning that urges a manual performance of a forced regeneration is given to an operator.
Herein, Patent Literature 1 describes that an exhaust gas amount adjustment unit is included to reduce a rate of decrease of an amount of exhaust gas flowing in the DPF when a DPF temperature is raised by decreasing an amount of incoming air during the automatic forced regeneration, and the DPF is prevented from being overheated.